Conventionally, there is known a technology of generating plasma by means of an electromagnetic wave using thermal electrons as a trigger. For example, Japanese Unexamined Patent Application, Publication No. 2009-287549 discloses a compression ignition type internal combustion engine which this kind of technology is applied to. In this compression ignition type internal combustion engine, thermal electrons are emitted from a surface of a glow plug. A microwave is radiated to a region where the thermal electrons are provided. Then, the thermal electrons receive energy of the microwave and are accelerated. The accelerated thermal electrons collide with ambient molecules and ionize the molecules. Electrons emitted by the ionization are also accelerated by the microwave and collide with ambient molecules and ionize the molecules. Thus, ionization of the molecules by the microwave forms an avalanche, thereby generating microwave plasma.